A waveguide is a structure that guides waves, such as electromagnetic waves or sound waves. They enable a signal to propagate with a very small loss of energy by restricting expansion to one dimension or two. This is a similar effect to waves of water constrained within a canal, or why guns have barrels that restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, signals will typically dissipate according to the inverse square law as they expand into three dimensional space. There are different types of waveguides for each type of wave. The original and most common is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves.